This invention relates to electronic article surveillance (EAS) systems and, in particular, to EAS systems which utilize processing of received signals.
U.S. Patent 4,859,991, assigned to the same assignee hereof, discloses an EAS system of the magnetic type in which a low frequency magnetic signal or field at a preselected transmitter frequency is transmitted into an interrogation zone. If a magnetic tag is present in the zone, the tag interacts with the transmitted field to cause perturbations in the field at harmonics of the transmitter frequency.
Magnetic fields are received by the system from the interrogation zone and processed by a front-end processor to remove or extract interference signal content in the received signals. The resultant front-end processed signal is then further analyzed or processed via a tag evaluation processor to determine whether the signal contains any tag signal content. If the analysis indicates the presence of tag signal content, an alarm is sounded to indicate that a tag is present in the interrogation zone.
In the '991 patent, various types of interference signals are extracted by the front-end processor. One type of interference results from the power line signal used to supply power to the system components and other equipment adjacent to the interrogation zone. In the '991 patent system, the front-end processor extracts this interference via a comb notch filter having rejection bands at the power line frequency and its harmonics.
Another type of interference present in the '991 patent system is that originating from the interaction of the transmitted magnetic field with the magnetic shielding used to confine the transmitted field to the interrogation zone. This shielding results in so-called "shield-spike" interference in the received signals. Shield-spikes occur at the peaks of the transmitted field and, thus, are spaced in time at one-half the period of the transmitted field.
The processing in the '991 patent system eliminates shield-spike interference by utilizing blanking. Blanking blanks out the front-end processed signal over blanking periods which occur at the peaks of the transmitted signal. Thus, during the blanking periods, no signal is processed by the tag evaluation processor and, therefore, such processing is unaffected by the shield-spike interference.
In the '991 patent system, the front-end processed signals are conveyed to the tag evaluation processor during a window which precedes each blanking period. Each window occurs at a zero cross-over point of the transmitted field. During each blanking period, the evaluation processor processes frequency and time domain information received from the front-end processor during the preceding window. The system of the '991 patent, thus, operates in successive frames each formed by a window and blanking period which together cover one-half the period of the transmitted field.
Also, in the '991 patent system, the system utilizes two transmitter antennas which are driven at 180.degree. out of phase relative to one another. This results in shield spikes in the received signals from the two transmitted signals which occur at substantially the same time. As a result, the same blanking periods and windows can accommodate the received signals resulting from the two transmitted signals.
U.S. Pat. No. 4,975,681, also assigned to the same assignee hereof, discloses a technique for improving the front-end processor of the '991 patent system. In particular, the '681 patent discloses a technique which when used in the '991 patent system is capable of removing both the power line and shield spike interference from the received signals. In the disclosed technique, the drive signal establishing the drive for the transmitter antennas is locked or synchronized in time with the power line signal while a time delay filter having a delay related to the period of the power line signal is used to filter the received signals. By suitable selection of the time delay, the power line and the shield-spike interference in the received signals is rejected and not passed by the filter, while the tag signal content is allowed to pass for a finite period of time.
The '991 patent system as modified by the '681 patent technique has certain limitations. First, the 180.degree. phase difference between the drive signals of the transmitter antennas results in regions in the interrogation zone, particularly, in the middle of the zone between the two antennas which have little or no resultant field in the horizontal direction. This limits the ability of the system to detect tags oriented in this direction. Also, in systems where the system is transmitter field limited, the use of a blanking period during each processing frame limits the detection at a given interrogation zone width. It also limits the interrogation zone width over which tags can be detected. Additionally, the use of a blanking period increases the overall transmit field requirements for a given zone width.
Furthermore, in the '991 patent system, two receiver antennas are used. These antennas are placed in series or parallel to best combine the received signals from the two antennas. However, this still results in cancellation of tag signal content when opposite polarity tag signals are combined.
It is, therefore, an object of the present invention to provide an EAS system of the '991 patent type which does not suffer from the above disadvantages.
It is a further object of the present invention to provide an EAS system of the '991 patent type in which the system can have a wider interrogation zone, more effective detection for a given width of the zone and reduced transmit field requirements for a given width of the zone.
It is yet a further object of the present invention to provide an EAS system of the '991 patent type in which the system is less prone to cancellation effects which result when the received signals from multiple receiver antennas are combined.
It is also an object of the present invention to provide an EAS system of the '991 patent type in which the system utilizes multiple transmitter antennas and is operated so as to provide field components in all directions in the interrogation zone.